DETECTING NONLINEARITY IN A CABLE PLANT AND DETERMINING A CABLE LENGTH TO A SOURCE OF THE NONLINEARITY

• US 20100070228A1
• Filed: 07/31/2009
• Published: 03/18/2010
• Est. Priority Date: 09/17/2008
• Status: Active Grant

• Current Assignee: Viavi Solutions, Inc. (Lumentum Holdings Inc.)
• Original Assignee: Daniel K. Chappell, Acterna LLC (Lumentum Holdings Inc.)
• Inventors: CHAPPELL, Daniel K.
• Granted Patent: US 8,548,760 B2
• Time in Patent Office: Days
• US Class Current: 702/79
• CPC Class Codes: G01R 31/58 Testing of lines, cables or conductors testing of electric windings G01R31/72
• CPC Class Codes: H04N 21/44209 Monitoring of downstream path of the transmission network originating from a server, e.g. bandwidth variations of a wireless network arrangements for maintenance or administration in data switching networks involving bandwidth and capacity management H04L41/0896
• CPC Class Codes: H04N 21/44245 Monitoring the upstream path of the transmission network, e.g. its availability, bandwidth
• CPC Class Codes: H04N 21/6118 involving cable transmission, e.g. using a cable modem
• CPC Class Codes: H04N 7/173 with two-way working, e.g. subscriber sending a programme selection signal

Abstract

An apparatus and a method for detecting a nonlinearity in a cable plant and for determining cable length to a source of the nonlinearity are disclosed. Upstream signal peaks are detected by averaging upstream signal waveforms. The upstream signal peaks are generated at the source of the nonlinearity from naturally occurring downstream signal peaks propagating in the cable plant. The downstream signal peaks occur due to constructive superposition of the downstream channel signals. Acquisition of the upstream signal waveforms is triggered by the downstream signal peaks. The cable length to the source of nonlinearity is determined from a time delay between the downstream signal peaks and the upstream signal peaks.

Claims

1. An apparatus for determining a cable length between a test point and a source of nonlinearity in a cable plant for propagating upstream and downstream signals, the apparatus comprising:

a peak detector for providing a triggering signal upon detection of a downstream signal peak at the test point of the cable plant;

a waveform acquisition unit coupled to the peak detector, for acquiring, upon receiving the triggering signal from the peak detector, a waveform of the upstream signal at the test point; and

a processing unit coupled to the waveform acquisition unit, for averaging waveforms acquired thereby, to obtain an averaged waveform of the upstream signal;

for detecting an upstream signal peak in the averaged waveform of the upstream signal; and

for determining the cable length between the test point and the source of nonlinearity, from a time delay between the downstream and the upstream signal peaks.

2. An apparatus of claim 1, wherein the peak detector has a comparator for comparing an amplitude of the downstream signal to a pre-defined threshold value, and for providing the triggering signal to the waveform acquisition unit when the downstream signal amplitude exceeds the pre-defined threshold value.

3. An apparatus of claim 2, wherein the comparator includes a circuit for detecting positive downstream signal peaks, and a circuit for detecting negative downstream signal peaks.

4. An apparatus of claim 1, wherein the waveform acquisition unit includes an analog to digital converter (ADC) for digitizing the upstream signal at the test point, and a memory buffer coupled to the ADC, for storing the waveform of the upstream signal upon receiving the triggering signal from the comparator.

5. An apparatus of claim 4, wherein the ADC has a sampling frequency that is at least twice higher than a highest frequency of the upstream signals.

6. An apparatus of claim 1, wherein the processing unit includes an averaging unit for averaging waveforms acquired by the waveform acquisition unit.

7. An apparatus of claim 6, wherein the processing unit further includes a calculation unit suitably programmed for detecting an upstream signal peak in the averaged waveform of the upstream signal, and for determining the cable length between the test point and the source of nonlinearity, from a time delay between the downstream and the upstream signal peaks.

8. An apparatus of claim 1, wherein the cable plant is a coaxial cable plant of a hybrid fiber-coax network having an optical fiber link operationally coupled to the coaxial cable plant, and wherein the apparatus further comprises a delay unit disposed between the peak detector and the waveform-acquisition unit, for delaying the triggering signal by a delay time corresponding to a time of propagation of a signal in the optical fiber link.

9. An apparatus for detecting a common path distortion signal in a cable plant for propagating downstream channel signals in a downstream spectral band and upstream channel signals in an upstream spectral band, the apparatus comprising:

a comparator for providing trigger pulses upon detecting high-magnitude downstream electrical pulses due to a constructive superposition of the downstream channel signals in the downstream spectral band, by comparing an electrical signal amplitude in the downstream spectral band to a pre-defined threshold value, and by generating a trigger pulse when the signal amplitude in the downstream spectral band exceeds the pre-determined threshold value;

an analog to digital converter (ADC) for digitizing, at a clock frequency, an electrical signal in the upstream spectral band;

a memory buffer coupled to the ADC, for storing, upon receiving the trigger pulses from the comparator, waveforms of the digitized electrical signal; and

an averaging unit coupled to the memory buffer, for averaging the waveforms stored in the memory buffer, so as to obtain an averaged waveform of the electrical signal in the upstream spectral band, wherein in operation, the averaged waveform has a peak corresponding to common-path distortion signals induced by the high-magnitude downstream electrical pulses detected by the comparator.

10. An apparatus for determining a cable length between a test point of a cable plant and a source of common path distortion in the cable plant, the apparatus comprising:

an apparatus for detecting a common path distortion signal of claim 9, wherein the electrical signals are at the test point; and

a calculation unit coupled to the averaging unit, for determining a position of the peak in the averaged waveform, and for determining the cable length between the test point and the source of common path distortion, from the determined position of the peak in the averaged waveform.

11. An apparatus of claim 10, further comprising a trigger controller for providing a polarity signal,

wherein in operation, the comparator detects positive or negative high-magnitude downstream electrical pulses, depending upon the polarity signal provided to the comparator;

and

wherein in operation, the calculation unit determines the position of a positive or a negative peak in the averaged waveform, depending upon the polarity signal provided to the calculation unit.

12. An apparatus of claim 10, further including a delay unit disposed between the comparator and the memory buffer, for delaying the trigger pulses provided by the comparator to the memory buffer.

13. An apparatus of claim 12, wherein the calculation unit includes:

a calculation circuit suitably programmed for determining the cable length between the test point and the source of common path distortion, and for determining the strength of the common path distortion signal,

wherein the calculation circuit is coupled to the averaging unit, and wherein in operation, the strength of the common path distortion signal is determined from a height of the peak in the averaged waveform; and

a display unit coupled to the calculation circuit, for displaying the cable length between the test point and the source of common path distortion determined by the calculation circuit, and for displaying the strength of the common path distortion signal determined by the calculation circuit.

14. A method for detecting a common path distortion signal in a cable plant for propagating upstream signals in an upstream spectral band and downstream signals in a downstream spectral band, the method comprising:

(a) comparing a signal amplitude in the downstream spectral band to a pre-determined threshold value, and generating a triggering pulse when the signal amplitude exceeds the pre-determined threshold value;

(b) acquiring a waveform of a signal in the upstream spectral band, upon triggering by the triggering pulse generated in step (a);

(c) repeating steps (a) and (b);

(d) averaging the waveforms acquired, to obtain an averaged waveform of the signal in the upstream signal band; and

(e) detecting a common path distortion signal, by detecting a peak in the averaged waveform.

15. A method of claim 14, wherein the downstream signals comprise a sum of downstream information channel signals transmitted over the cable plant, so that in step (a), the signal amplitude exceeds the pre-determined value as a result of a constructive superposition of the downstream information channel signals.

16. A method of claim 14, further comprising repeating step c) at least 500 times.

17. A method for determining a cable length between a test point of a cable plant and a source of common path distortion in the cable plant, the method comprising:

(i) detecting a common path distortion signal according to the method of claim 15, wherein in step (a), the signal in the downstream spectral band is a signal detected at the test point; and

(ii) determining the cable length between the test point and the source of common path distortion from a position of the peak in the averaged waveform.

18. A method of claim 17, wherein in step (ii), the position of the peak in the averaged waveform is denoted by an integer index n_PEAK, and the cable length is determined by multiplying n_PEAK by a constant C₁ to obtain a product, and by adding a constant C₂ to the product.

19. A method of claim 18, wherein the constant C₂ is selected so as to account for signal propagation delays caused by one or more selected from the group consisting of reactive modules, active modules, and cable spans of the cable plant.

20. A method of claim 17, wherein the cable plant is a coaxial cable plant of a hybrid fiber-coax network having an optical fiber link operationally coupled to the coaxial cable plant, wherein in step (a), the triggering pulse is delayed by a pre-determined delay time, to account for a signal propagation delay in the optical fiber link, and wherein in step (ii), the cable length is determined from a time delay between the triggering pulse delayed in step (a) and the peak in the averaged waveform.